The present invention relates to a method and a device for reducing vibrations of a control chain in a camshaft drive of an internal combustion engine by means of a chain tensioner.
It is known in the prior art to use an electronic control apparatus for belt tensioners in automotive vehicles. Such endless belts are employed for driving accessory systems such as a water pump, a power steering pump, an alternator, a refrigerant compressor, and the like. One such control apparatus is described in U.S. Pat. No. 4,478,595 by Hayakawa et al. In the method and device of U.S. Pat. No. 4,478,595, a belt tensioner is mounted on an upper portion of the engine and engages the belt at a location between two driving pulleys for accessory systems under control of an electric control apparatus. The belt tensioner uses a stepper motor driving a screw shaft to displace an idler pulley essentially in a direction transverse to the running direction of the belt. By the displacement of the idler pulley, the tension in the belt is determined. In addition, the idler pulley is pressed against the belt with a compression spring, which is also displaced with the stepper motor together with the idler pulley and also contributes to the tensioning of the belt. Furthermore, the belt tensioner comprises a displacement sensor to pick up the displacement of the idler pulley. The displacement sensor is connected to the electronic control apparatus.
The electronic control apparatus of U.S. Pat. No. 4,478,595 comprises a microcomputer and an A/D converter. It is connected to sensors for determining the power put into the accessory systems, such as an ammeter for an alternator, a pressure sensor for the power steering system, an ammeter for the refrigerant compressor clutch, and to other sensors, such as a speed sensor to determine the actual rotational speed of the engine crankshaft and the displacement sensor in the belt tensioner.
The sensors, the electronic control apparatus, and the belt tensioner described in U.S. Pat. No. 4,478,595 are combined to form a control system to optimally set the tensioning force in the belt so that an optimum power transmission is effected and the durability of the belt is enhanced.
There are several problems with the belt drive disclosed in U.S. Pat. No. 4,478,595. First, special sensors are needed to furnish the parameters used for controlling the belt tension. These parameters are not part of customary electronic engine systems and in particular may not be used for controlling the tension of drives other than an accessory drive. Second, the belt does not exhibit a high mechanical strength if it is subjected to high temperatures. In modern engines, however, the thermal load of drive systems used therein has increased considerably, due to the lack of space, which is a problem to the provision of sufficient heat sinks. Finally, the drive shown in U.S. Pat. No. 4,478,595 is not concerned with the reduction of the vibrations generated by the belt drive. The vibrational characteristics of the belt drive of U.S. Pat. No. 4,478,595, however, do not meet the requirements for today""s smoothly running automotive engines and their decreased vibrations and sound emissions.
To meet the demands on the thermal stability of the drives used in modern engines, chain drives, for instance for camshaft drives, are now in general use in many internal combustion engine constructions. For example, in an internal combustion engine having two overhead camshafts the chain is guided via a chain gear connected to the crankshaft and via the two adjacent camshaft chain gears.
To reduce the vibrations of the chain as much as possible, the chain is normally guided at one side along a stationary slide rail of a suitable sliding material. Opposite to said stationary slide rail is arranged a movable contact-pressure rails which is pressed by means of a chain tensioner onto the chain.
There are many different chain tensioner designs, which are to meet the different demands made on their use. Other chain tensioners are for instance known whose tensioning force is applied with the help of an expansion element for compensating for temperature variations. As a rule, the chain tensioners are based on the principle of combining a tension applied by means of a spring element and a damping action performed by means of a hydraulic construction. It is also known that the damping means is coupled with the engine oil circuit to achieve a stronger damping action at higher speeds. The tensioning force, which is predetermined by the spring means, is thereby only influenced to a limited extent.
An example of a self-adjusting belt tensioners that uses both a spring element and a self-adjustable hydraulic piston is known for example from U.S. Pat. No. 5,637,047, which is incorporated herein by reference.
In the chain tensioner of U.S. Pat. No. 5,637,047, a tensioner for a chain-type driving member includes a piston slidably received in a pressure chamber of a housing and being biased by a spring in the pressure chamber in a direction out of the housing against a chain. The pressure chamber is connected to the hydraulic circuit of the engine via a backpressure valve that is closed by the pressure in the pressure chamber. If the tension in the chain decreases, the piston is moved by the spring out of the housing, thus increasing the volume of the pressure chamber, reducing the pressure therein and opening the backpressure valve. In this way, hydraulic fluid is supplied to the pressure chamber to adjust the piston. To avoid high pressures that may damage or wear the chain, a relief-valve is also provided. The relief valve opens if a predetermined pressure is exceeded in the pressure chamber.
The tensioner of U.S. Pat. No. 5,637,047 has been optimized to decouple the resonant frequencies of the backpressure valve and the relief-valve from the resonant frequencies of the motor, which previously affected the operation of such valves. The tensioner disclosed in U.S. Pat. No. 5,637,047 needs, however, further improvement in its damping characteristics if the chain exhibits large vibrations. Another type of chain tensioners, such as for example known from U.S. Pat. No. 6,193,623, uses the discharge of hydraulic fluid such as engine oil from a pressure chamber via at least one leak gap to dampen the vibrations. This method, however, also needs to be improved in view of high vibrations of the chain at selected frequencies or at particular operational conditions.
In view of the prior art, it is therefore the object of the present invention to provide a method and a device of the above-mentioned type, which improves the vibration behavior of a chain control chain in a camshaft drive of an internal combustion engine.
This object and others, which will become apparent hereinafter, are attained in accordance with the invention by determining a vibrational state of the combustion engine by measuring at least one engine parameter, and adjusting the tensioning force generated by a chain tensioner depending on the determined vibrational state to reduce said vibrations of said control chain.
The term engine parameter is used for a parameter that is used to define the operational state of the engine and to control the operation of the engine, such as the engine torque, the engine speed, the engine and oil temperatures, the mixture of the air-fuel mixture and knocking and misfire signals. It is essential to the invention that an engine parameter is used that represents the vibrational state of the camshaft drive and of the engine. The vibrational state of these systems is defined by the frequency components of the vibrations, at least by a subset of these frequency components, e.g. the strongest components. As the vibrational state of the engine determines the possible excitation of resonances of the control chain, the tensioning force is adjusted depending on the vibrational state of the engine. Due to the adjustment of the tensioning force, the vibrational properties of the camshaft drive are changed and the built-up of resonances is avoided. The tensioning force is adjusted by operating the chain tensioner under control of the electronic control system
This solution differs quite clearly from the solution as, for example, realized in U.S. Pat. No. 4,478,595, where the vibrational state of the engine neither can be determined nor plays a role in the adjustment of the tensioning force. Further, U.S. Pat. No. 4,478,595 does not use engine parameters, but parameters of the accessory equipment for determining the tension of a belt drive.
To determine the vibrational state of the engine, a predetermined relationship between the at least one engine parameter may be stored in the electronic control system. This predetermined relationship is a characteristic in the form of a table or a function or map. The characteristic may be used to determine the vibrational state of the engine by looking up a quantity indicative of the vibrational state of the engine depending on the at least one engine parameter. In particular, the at least one engine parameter may comprise at least one of the engine speed and the engine torque.
As a result, the device and method according to the invention actively influence the vibration behavior, for instance, of the chain drive, thereby ensuring an optimum chain tension at any given operational time. The tensioning device must be constructed such that it can actively adjust the tensioning force on the basis of control signals. The force adjusting operation as such can be carried out electrically or e.g. hydraulically, depending on the respective application.
To keep the number of additional components as small as possible in comparison with conventional internal combustion engines, it is useful according to a further embodiment that the control unit is connected to and/or integrated into the electronic engine system of the internal combustion engine that supplies the sensor data. As a rule, modern automotive vehicles are nowadays equipped with an engine control unit, which already determines all operational data, in particular with respect to the characteristic map. The vibration behavior of the engine can directly be inferred from said values: In response to said data the chain tensioner for instance, can then be controlled. Thus, a chain tensioner construction would be conceivable that is just connected to the electronic engine system by means of supply lines, whereby an automatic coupling with suitable sensors taken place.
To react as quickly as possible at any time and to permanently monitor the tensioning force it makes sense to use a control loop. To this end a control means may be provided for controlling the tensioning force in response to the sensors. Furthermore, it may be necessary to measure either the applied force, for instance, of the chain tensioner as such or the tensioning distance covered (or relative path) of the chain tensioner.
Although the tensioning device can also be operated pneumatically, hydraulically, mechanically or thermally (e.g. by means of an expansion element), the chain tensioner is actuable in one embodiment by means of an electric adjusting device for adjusting the tensioning force. The sensor signals can thus be used for directly controlling such an electric adjusting or lifting mechanism.